1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a semiconductor device wherein, while avoiding the re-deposition of the native oxide film once removed by etching, the electric characteristics of the device is prevented from being lowered.
2. Background Art
When the parasitic capacitances of a diffusion resistance layer in a source/drain region and a gate electrode resistance increase as a result of the scale-down and the high degree of integration of a semiconductor device being in progress, signal transmission is delayed thereby impeding high-speed operations. In order to realize a high-speed device by reducing the parasitic capacitance, a technique has been adopted wherein a source/drain region and a gate electrode region are self-alignmently converted to a metal silicide.
FIGS. 7 and 8 are views each showing an example of the conventional step of forming a cobalt silicide film. In FIG. 7(a), agate insulating film 23 made of a silicon oxide film, a gate electrode 24 made of a polysilicon film, and a side wall 25 made of a silicon oxide film are formed on a silicon substrate 22.
Since the surface of silicon is active, a native oxide film 26 naturally formed in air exists (FIG. 7(b)). To cope with this, as shown in FIG. 7(c), plasma etching is carried out by use of an argon (Ar) gas so that the native oxide films 26 formed on the surfaces of the silicon substrate 22 and the gate electrode 2A are removed. Subsequently, after deposition of a cobalt (Co) film (as 27 in FIG. 8(b)) by a sputtering method, conversion into a silicide by thermal treatment, selective etching and annealing are carried to form a cobalt silicide (CoSi2) film (as 28 in FIG. 8(c)).
As a result of intensive studies made by the present inventors, it has been found that the above-stated silicide film-forming process has the following problems.
When the native oxide film 26 is removed by physical sputtering using an argon gas prior to the deposition of the cobalt film 27, a residue 29 of the etched native oxide film is deposited on the sidewalls 25 as shown in FIG. 8(a). The silicon oxide (SiO2) constituting the native oxide film 27 is decomposed into silicon (Si) by the action of a plasma made of the argon gas, and thus, the resulting residue 29 contains silicon aside from silicon oxide. Accordingly, when the cobalt film 27 is formed thereon, the silicon in the residue 29 reacts with cobalt to form the cobalt silicide 28 on the sidewall 25 as is particularly shown in FIG. 8(b).
Because silicon oxide or silicon nitride constituting the sidewalls does not react with cobalt, cobalt silicide is not formed at the sidewall in the nature of things. However when a residue of plasma etching is deposited on the sidewall, cobalt silicide is formed on the sidewall, with the attendant problem of causing electric short-circuiting.
In Japanese Patent Laid-open No. Hei 5-267207, attention has been paid to the problem caused by plasma etching with use of an argon gas, and an etching method using a reactive gas in addition to an argon gas is disclosed. However, this publication deals only with the problem of damaging a silicon substrate with etching wiring an argon gas, and no mention is made of the deposition of a residue after etching.